fix: reset viewport on window resize to prevent content jump and loss

[CoderGoGoGo]

Summary

• Reset all pane viewports to bottom after every window resize in apply_dimensions(), preventing content from appearing to jump or becoming inaccessible
• Limit pre-prune blank line removal in Screen::resize() to avoid cumulative content loss during repeated resizes

Problem

When resizing the terminal window:

1. The last line of content jumps upward
2. Cannot scroll up to see previous content
3. Repeated resizes progressively clear terminal content

Root Cause

physical_top shifts on resize because both lines.len() and physical_rows change, but PaneState::viewport was never adjusted. Additionally, the pre-prune loop in Screen::resize() had no upper bound, causing cumulative content loss.

Test Plan

• Output several screenfuls of content, then resize the window — content should not jump
• After resize, scroll up — all previous content should be accessible
• Rapidly resize multiple times — content should not be cleared

🤖 Generated with Claude Code

[tw93]

@CoderGoGoGo 感谢 PR，问题定位是对的。调整了一下 viewport 的处理方式：resize 后对已有 viewport 做 normalize 而不是直接重置为 None，这样用户在查看历史内容时 resize 不会被拉回底部。pre-prune 的上限逻辑保留了你的思路。已合入 main。